Various curing agents are known in the art for use in epoxy resin systems, such as aliphatic polyamines, polyamides, amido-amines, cycloaliphatic amines, certain anhydrides, phenol-formaldehyde, urea-formaldehyde, melamine-formaldehyde and aromatic amines.
Aromatic diamines have been in use for many years as curing agents for epoxy resins. They were originally introduced to increase the glass transition temperature (Tg) of the cured resins in order to impart increased dimensional stability at elevated temperatures. In addition, aromatic diamines have been found to provide long pot lives. The cured epoxy resins produced using aromatic diamines have good physical properties and good resistance to solvents and chemicals.
The aromatic diamines most widely used for curing epoxy resins are m-phenylene diamine (MPD), methylene dianiline (MDA) and diaminodiphenylsulfone (DADS). In the aerospace industry, DADS is a preferred curing agent in producing epoxy composite matrix resins due to the 30.degree. C.-40.degree. C. temperature increase in the heat distortion temperature which is imparted when DADS is utilized as compared to the temperature achieved with either MPD or MDA. In other applications, however, MPD or MDA are preferred because they are less expensive and require a less energetic cure schedule. Further, MPD and MDA are preferred since liquid curing agents are normally preferred over solid curing agents. MPD and MDA are commonly marketed as an eutectic mixture which is liquid at room temperature. Recently, however, MDA and MPD have come under an increasing attack due to their high toxicity. In fact, MDA has been found to produce cancer in laboratory rats. Accordingly, a need has arisen for new aromatic diamines which can serve as replacements for MDA and/or MPD/MDA eutectic mixtures without causing detrimental effects in the cured epoxy resins.
One means discovered for lowering the toxicity of aromatic diamines is to introduce ring substituents which can be readily metabolized, such as to a carboxylic acid. One example of this type of curing agent is trimethylene glycol di-p-aminobenzoate which, upon ingestion, is hydrolyzed to produce a non-toxic p-aminobenzoic acid. Due to the drastic departure from the basic MDA or MPD structure, however, changes in the performance properties of the cured epoxy resins have occurred thereby making the amino-benzoate an unsatisfactory curing agent for many applications.
In an article entitled "Properties Of Epoxy Resins Cured With Ring-Alkylated M-Phenylene Diamines", ACS Symp. Ser. 367, Ch. 14, pgs. 182-198, R. F. Storey et al (1988), the curing of epoxy resins with individual ring-alkylated MPDs was reported. These compounds are believed to be less toxic than MDA or MPD due to the presence of the alkyl group which is readily oxidizable in the body to produce the diamino benzoic acid. The individual ring-alkylated MPDs considered in the article were toluene diamine (TDA), diaminoethylbenzene (DAEB), diaminoisopropylbenzene (DAIPB), and diamino-tertbutylbenzene (DATBB). The alkyl group was also found to impart a significant lowering of the melting point as compared with that of MPD. For example, DATBB is a liquid and DAIPB is a soft solid at room temperature. Branched alkyl groups are preferred since they are suitable for use in Friedel-Crafts alkylation reactions and since they are less likely to cause internal plasticization in the cured resins. Larger alkyl substituents, such as isopropyl or tert-butyl, were found to increase the Tg of the cured resins. The most significant disadvantage in using the ring-alkylated MPDs was a 30%- 35% loss of tensile strength in the cured resins as compared to that of MPD. It was shown that this loss was not due to an incomplete cure, which could conceivably have been caused by the steric bulk of the ring substituent, but rather appears to be due to the physical presence of the large alkyl group within the crosslinked network.
Due to the several disadvantages associated with the ring-alkylated MPDs, alternatives have been sought to increase the tensile strength of cured resins. Accordingly, it has been discovered that certain blends of ring-alkylated diamines combine synergistically to yield cured resin tensile strengths comparable to MPD and much higher than either ring-alkylated diamine when used alone. In particular, various blend ratios of DAIPB and DAEB have been found to provide cured epoxy resins which display an increased tensile strength and slightly lower glass transition temperatures as compared to epoxy resins cured with either DAIPB or DAEB alone. Blends of DAIPB and DAEB are also advantageously present as eutectic liquids at room temperature.